X-ray Computerized Tomography (CT) is currently the only automated explosive detection technique for the aviation industry that has been certified by the United States Government. The technique involves taking x-ray images at many different angles through an object and using mathematical reconstruction algorithms (such as filtered back-projection) to reconstruct tomographic slices through the object. These slices can then be combined to form a three-dimensional model of the object. In particular, the method allows the density of organic materials concealed inside the container to be determined, along with their size, volume, shape, and mass. This information can then be used to automatically detect explosives by looking for objects above a minimum size and mass, which have a density in the range of about 1.1-1.8 grams per cubic centimeter.
When used to screen checked baggage at airports, the typical false alarm rate for X-ray CT systems is between about 20% and 35%. It is therefore quite likely that for a typical air cargo container, several items in the container will have a density that lies in the density range of explosives, and false alarms would thus overwhelm airport cargo flow.